Archive for April 11th, 2012

Well, finally found a work around for the 1.4GB limit imposed on the V8 heap. This limitation was at first a “hard” limit, and after some tweaks it became a “soft” limit. Now, the combined tweaks listed below have removed the limit entirely! YESSSSSS!!

The first two were already mentioned in my previous few blog articles, and the new ones (#3 and #4) finally open up the possibility of utilizing the real capacity of your server hardware.

1) ulimit -n 999999

This effectively increases the number of sockets Node.js can have open. You won’t get far without it, as the default tends to be around 1024.

2) –nouse-idle-notification

This is a command line parameter you can pass to node. It gets passed along to the V8 engine, and will prevent it from constantly running that darn garbage collector. IMO, you can’t do a real-time server with constant ~4 second latency hiccups every 30 seconds or so.

You might want to also use the flag “–expose-gc”, which will enable the “gc();” function in your server JavaScript code. You can then tie this to an admin mechanism, so you will retain the power to trigger garbage collection at any time you want, without having to restart the server. For the most part, if you don’t leak Objects all over the place, you won’t really need to do this often, or at all. Still, it’s useful to have the capability.

3) –max-old-space-size=8192

This you can tweak to fit your particular server, but the value is in MB. I chose 8GB because my expectation is that 4GB is going to be plenty, and 8GB was just for good measure. You may also consider using “–max-new-space-size=2048” (measured in KB, as opposed to the other). I don’t believe that one is nearly as critical, though.

4)Compile the latest V8 source code, with two modifications

The current node distribution isn’t using the new V8 engine which has significant improvements to garbage collection and performance in general. It’s worth upgrading, if you need more memory and better performance. Luckily, upgrading is really easy.

The second line above is new. Basically, you just need to set that V8_MAX_SEMISPACE definition. It will otherwise default to a much lower value, causing frequent garbage collection to trigger, depending on the memory characteristics of your server JS.

This was done “just in case” because it costs me money each time I run a scaling test. I wanted to be damn sure it was not going to trigger the external allocation limit GC!

That’s it! You will now want to rebuild both V8 and node, I would recommend doing a clean build for both, since replacing the entire V8 source may otherwise fail to take effect.

While writing this, I’ve got a server log open with 2281 MB of V8 heap usage. That’s far beyond the normal 1.4 GB limitation. The garbage collection is behaving, the server remains VERY responsive and low latency. In addition to the 250k concurrent and active connections, each of those nodes is also sending a sprite every 5 seconds.

There is still CPU to spare, the only limitation keeping me from trying 500k concurrent is my Amazon EC2 quota, which I already had to request an upgrade to do 250k.